1. Field of the Invention
The present invention is directed to a method and apparatus for a material handling feed table wherein movement of a reciprocating shuttle device is coordinated with movement of driven rollers.
2. Prior Art
Many industries require solid materials to be delivered to or taken from a process machine or equipment. One example would be a machine operation such as cutting a length of material on a band saw. The material may be a single piece, such as a large ingot, or a bundle of pieces weighing up to 20,000 lbs. This work is often accomplished by the use of some type of conveyor, a mechanism that moves materials with the aid of rollers, bearings, or other anti-friction devices built into its structure. A conveyor saves energy as those anti-friction devices help move the mass of the material, while more or less rigid structure supports the material's weight.
A conveyor is often called by a descriptive name. For instance, a conveyor used to carry away the results of a process might be called a discharge conveyor, product conveyor, or the like. Similarly, a conveyor used to carry material towards a process might be called an input or in-feed conveyor.
It is also common to add a descriptive term regarding the construction to the name of the conveyor. One example of a descriptive name is the so-called feeding table, or feed table.
The present invention is directed to a feed table conveyor. This device is called a table because, in the simplest form, it has a more or less horizontal top surface that the supports the material placed upon it. Once the table has been loaded, the material can be translated or moved over the table's surface towards the process.
One of the simplest types of feeding tables is known as a roller table. Rollers are cylindrical structures, having a main axis running concentric to the curved outer shell and through the centers of the circular ends. The construction may vary but the strength of the roller must support the load that it is to move. The weight of the material on a roller is supported by surface of the rollers pushing up at the points of contact. This roller is most often mounted on an axle or shaft coaxial to the main axis and the curved outer shell. It is around this axle that the shell revolves during use. The axles transfer the forces applied roller to the supporting structure through bearings, which not only anchor the roller in place but also permit the shell to rotate.
A roller table is an array of rollers with their axles parallel to each other. The rollers are positioned so that the top of each roller is coplanar, that is to say that they are tangent to a single plane. This upper tangent plane between the rollers defines the top of the table and is more or less horizontal. Material placed on this conveyor can translated freely in only one direction, parallel to the top plane and transverse to the roll axis. The material is moved while the supporting rollers rotate about their axles.
One example of a roller table is seen in Assignee's U.S. Pat. No. 5,353,910 for a Shuttle Vise Assembly for a Feed Table Apparatus.
Movement is made easier if the material to be processed has at least one more or less planar surface. In use, the material is turned to place the planar surface against the feed table's top surface, or if there is no suitable planar surface, the material can be placed on or anchored to a carrier, which provides a smoother surface to help the material pass over the table. The carrier can be modular, like a machining jig, or continuous, like a conveyor belt.
The roller table is particularly suited for moving material of undefined length and a more or less constant width or cross-section, such as a beam. The material can be translated axially down any length of table, while the length of the main axis of the rollers need only to be just longer than the width of the material, so it is completely supported. The roller array forming the top of the table can extend as far as needed to carry the material through the process. The conveyor can be built as a single roller array supported by one continuous structure or a collection of roller arrays, aligned to be collinear and coplanar, and supported by independent structures. Because of the linear configuration of this roller array, it is often called a line and is often constructed as a series of smaller roller tables.
The material may be pushed or pulled down the length of a roller table, with idling rollers only turned by the passing contact of the material. In this manner, the rollers are only used to support the weight of the material and to reduce the moving force by reducing friction and nothing else.
Alternatively, it is often convenient to use the rollers themselves to drive the material by using a powered drive system to turn one or more rollers. The force to translate the material comes from traction derived from the friction between the rollers and the material surfaces. This method requires no special connection between the work pieces. The material is simply placed on the conveyor and held against the rollers by gravity. The location or number of pieces of material on the table does not affect the movement. Work pieces can be advanced into one process and the product carried away to another process on a continuation of the same line. The speeds of segments or zones along the length of the table can change independent of the speeds in other zones.
The main advantages of using a power roller conveyor as a feed table include ease of use and speed to load and unload. They can move material relatively quickly as the motion is continuous. The load is distributed along the length of the conveyor. If a more or less constant weight per unit length is maintained, so that a greater material mass results in a greater length, then weight is spread over more rollers and the local supporting load, the stress on any structural section remains more or less constant. Movement is limited by the power to overcome inertia and friction.
One drawback of roller conveyors is that they have limited control over the exact position of the material. Many processes require that the feeding tables deliver material to a specific working zone in precisely measured increments or to an exact position. As only the friction between the work piece and the powered rollers drives the material and that friction varies with surface condition, there is no sure relationship between the movement of the rollers and the movement of the material. This is partly because, the material travels along a more or less linear path, while the roller surface follows a circular path.
As a single point on the surface of the roller revolves about the center axis, its lateral velocity changes. When the point is near the top tangent point, the velocity is approximately the same as the passing material. When the point rolls towards the side, the lateral velocity decreases to zero. Then as the point passes the side and rolls towards the bottom its speed increases to equal its speed at the top but it is traveling in the opposite direction. The point then decelerates again and reverses direction to accelerate back to the material speed as it reaches the top of the roll.
In theory, the material velocity and the surface speed of the roller will be equal where they touch at a single tangent point. In practice, the exact speed and point of contact is uncertain. There are always some surface distortions that result from the contact forces, so the material surface is never plane and the roller surface is not truly cylindrical where they meet. The resulting variations in surface velocities cause the distorted surfaces at the interface to creep, first one way, then the other, as they pass. Also, at any time, there are small variations in speeds between different rollers, due to manufacturing tolerances of the drive components. Since the contact points of the transported material are linked together as a single more or less rigid body and there is some elasticity in all components, the velocity of the work piece becomes a rough average of the tangential velocities of all rollers in contact at the movement and the absolute position of the work piece uncertain.
One solution to the positioning problem is to employ a secondary measurement device, driven by the motion of the work piece. This, however, adds complexity and may be subject to some of the slippage issues previously described.
A more common method is to abandon the driven rollers and directly drive the material. Free rollers are still used to support the weight with the rollers turning passively as the material travel over them. This is often accomplished by using a traveling clamping device to anchor a driving mechanism to the work piece. This clamp is often called a shuttle vise as in practice, its traveling distance is limited and the vise must be repositioned, recycling back along the length toward its starting position when the clamp reaches the limits of its travel. The work piece must be held in position as the clamp moves to a new position before re-engaging.
One non-limiting example would be a band saw for cutting material into desired lengths. While the saw blade is cutting, the shuttle vise is released from the material and is moved back to the desired clamping position.
The vise firmly anchors the work piece to the drive mechanism, making its actual position more certain. The main disadvantage of the shuttle vise is that its motion cannot be continuous. If only one shuttle vise is used then the vise and the material must accelerate and decelerate, repeatedly, if the length of the piece to be fed exceeds the remaining travel length of the shuttle. The constant starting and stopping and the force of dragging material over rollers puts extra stress on the drive mechanism. If the work piece is heavy enough, these forces may create measurement inaccuracies or require additional materials and structures to re-enforce the shuttle.
The use of a single shuttle vise also slows the movement of long pieces of material as it requires a dwell time where the material stops and waits while the vise recycles. The motion control is complicated, but multiple shuttles can be used, by coordinating their movements, to allow any length material to pass without stopping. This is accomplished by working the vises in a hand-after-hand fashion, with one vise recycling while another continues to pull.
A problem, from an operational point of view, is that the material to be fed must be placed within reach of the shuttle system. Often a powered roller table can be used to bring new material to a point where the vise can clamp on to the piece. The problem is that as the shuttle clamps on and starts to pull the material forward, the rollers of the powered roller table tend to drag on the portion of the material that they still support.
The usual solutions to these problems have not always been satisfactory. Clutches can decouple the drive motors from the roller mechanisms, but the remaining component often will produce considerable drag. It is common that the tangential force that is required to turn the rollers may exceed the friction developed from the weight of the material applied to the surface of the rollers. In this state, the material will slide and drag over the rollers rather than be carried by turning rollers. Even when the rollers turn, the force of turning adds a considerable load to the shuttle while the material is in motion. They can be viewed as a series of small flywheels. Their rotational inertia causes them to resist acceleration and deceleration, effectively adding to the mass that the shuttle must move.
In addition, a traditional power roller system may contain a section that includes a common shuttle vise conveyor. An operator might wish to drive material quickly though this section without using the shuttle. Then, the un-powered section of rollers will not help move the material. In fact, the un-powered rollers can cause the material to stall if the traction force of the portion of the material supported by the powered rollers is not enough to drive it over the un-powered rollers. This often happens if the transported piece is short enough that it touches few or no powered rollers, or too few rollers to drive it. A cambered piece that does not set flat on the roller table can cause the same problem. The only way to continue to move the material is to stop using the powered rollers, and instead use the shuttle vise to move the piece.
Accordingly, it is an object and purpose of the present invention to provide a material handling feed table having both a series of driven rollers and a powered shuttle vise.
It is a further object and purpose of the present invention to provide a method or process to coordinate movement of a series of driven rollers with movement of a powered shuttle vise.
The present invention describes a feed table that includes the elements of both a powered roller table and at least one shuttling vise. All of the rollers, including the section where the shuttle vise or vises reciprocates, are powered. The roller conveyor can carry material quickly to the process equipment, when the clamping elements of the vises are withdrawn, since the roller array extends throughout the entire feed table.
When greater precision is required, the roller table can carry the work piece to the waiting shuttle vise system, but once the shuttle has clamped, the roller table mode changes to follow the motion of the shuttle. Instead of supporting the material on un-power rollers that passively follow the motion of the material, or require the material to simply slide if they don't turn free enough, these rollers are actively driven to follow the motion of the shuttle. This uses the ordinarily idle roller drive to eliminate the friction experienced by the shuttle vise, which in turn reduces the stress on the shuttle.
Additionally, the actively turning rollers would help lift the material as surface irregularities contact the rollers. When the contact surface of the material being moved is not smooth, these irregularities may drop below the tangent line of the rollers and into the void or voids between the rollers. As the material moves, these irregularities will sooner or later meet a roller surface. Once in contact, the roller will force this material back to the tangent line as the motion continues. Since the material is more or less rigid, this requires the roller to lift at least part of the material's weight. As the irregularity passes over the roller and continues on, the irregularity will drop into the next void. In shuttle systems without powered rollers, the extra power needed to lift an irregularity over a roller results in additional load retarding the motion of the shuttle. Conversely, as the irregularity drops into the void, the previously lifted weight tries to push the shuttle ahead. Irregularities occur randomly of any real surface, and these random loads are added to the existing friction and inertial loads to increase uncertainty.
It is a further object and purpose of the present invention to provide a material handling feed table wherein the speed of driven rollers is matched to the speed of a reciprocating shuttle.
An initial goal would be to match the shuttle speed, to minimize the difference between the speed of the transported material and the tangential surface speed of the roller. This should reduce and virtually eliminate the rolling friction, roller inertia, and reduce the effects of surface irregularities.
A motion control algorithm for the shuttle minimizes the pulling loads by smoothly accelerating and decelerating the shuttle and the load. The roller velocity should follow this same pattern. Once the motion program is determined then, as the shuttle starts to move, the actual position and velocity from the measurement system is used as feedback to insure that the program is accurately followed. This operation of error measurement and correction is continued throughout the motion program. If at any time the shuttle is moving too fast or too slow, the control will adjust the power to the drive powering the shuttle, to keep the motion on the program and deliver the work piece to its new position.
The roller drive will follow the commands of the shuttle but will also watch the actual speed of the rollers for any error between that and the shuttle's speed and adjust its output to match the speed and minimize slippage between the rollers and the material. The roller table will carry part of the load that results from surface irregularities. The traction between the material and a roller surface will help pull and lift the material, to ease their passage.
Since the roller conveyor drive is powerful enough to move the material by itself, when combined with the shuttle there is an excess of power available to move the material. The roller table can then share the work of moving the work piece, reducing the stress on and deformation of the shuttle vise and its associated measuring system, and reducing the uncertainty as the material is positioned in the processes work zone. The shuttle directly controls the motion of the material being moved by the roller table, providing only enough force to correct the motion, to improve the conveyors accuracy.